The present invention relates to an image forming apparatus by an electro-photography process such as a copying machine, a printer or a facsimile. More particularly, the present invention relates to an image forming apparatus which forms images utilizing an endless belt such as an endless intermediate transfer body.
Generally, an electro-photographic image forming apparatus (hereinafter referred to as an image forming apparatus) which utilizes an endless belt of an intermediate transfer body (hereinafter referred to as an intermediate transfer belt) to form color images using an electro-photography process is known in the art. In the image forming apparatus, a plurality of image forming units are disposed along an intermediate transfer belt and a second transfer roller is disposed at the downstream of the rotational direction of the intermediate transfer belt.
A conventional image forming apparatus is shown in FIG. 1. The conventional image forming apparatus has a plurality of image forming units 11-14, each respectively comprising a photosensitive body drum 11a-14a and a toner container 11b-14b. Image forming units 11, 12, 13 and 14 correspond to yellow (Y), magenta (M), cyan (C) and black (K), respectively. These image forming units, more specifically the photosensitive body drums, are disposed along the transporting direction of intermediate transfer belt 21 (a transporting pathway) so that photosensitive body drum 11a is disposed at the most upstream position of the transporting pathway and photosensitive body drum 14a is disposed at the most downstream position of the transporting pathway. Primary transfer rollers (not shown) are each disposed facing photosensitive body drums 11a-14a. 
Intermediate transfer belt 21 is supported by a driving roller 22a, a supporting roller 22b, a tension roller 23 and a backup roller 24. Intermediate transfer belt 21 is driven in the direction of the solid arrow head line shown in FIG. 1 by driving roller 22a, while being applied a predetermined tension by tension roller 23. Backup roller 24 faces a secondary transfer roller 25. A primary transfer part is defined between driving roller 22a and supporting roller 22b along which said image forming units 11, 12, 13 and 14 are disposed.
When images are formed, toner images are formed on photosensitive body drums 11a-14a synchronized with the movement of intermediate transfer belt 21. Primary transfer images are formed by primarily transferring the toner images on intermediate transfer belt 21. A recording medium (not shown) is sent from a paper supplying unit 27 to a secondary transferring position by being transported on a recording media transporting belt 29 with a paper reversing roller 28. The recording medium is sandwiched together with intermediate transfer belt 21 between backup roller 24 and secondary transfer roller 25, the toner images (primary transfer images) being secondarily transferred on the recording medium by secondary transfer roller 25. Subsequently, the recording medium is transported to a fixing device 30, where the images are fixed, and then discharged to a discharge tray 31. A recording medium may also be placed on the recording media transporting belt 29 by using manual inserting roller 28a. 
The image forming apparatus shown in FIG. 1 has a primary transfer part where intermediate transfer belt 21 is horizontally disposed, which results in the space occupied by the belt becoming unnecessarily large. Accordingly, volumes of toner hoppers, each storing a different colored toner, require frequent monitoring and equalization. For example, it is necessary to replenish black toner more frequently than other colored toners.
To avoid such drawbacks, an image forming apparatus is provided, for example, in Japanese laid-open patent publication JP1996-87151. In such apparatus, a transporting pathway for recording media, which are transported through a plurality of image forming units, is inclined with respect to the horizontal direction.
The former image forming apparatus having the aforementioned intermediate transfer belt 21, driving roller 22a, supporting roller 22b, tension roller 23, backup roller 24 and the primary transfer roller are fixed to an intermediate transfer belt supporting frame to compose an intermediate transfer belt unit. Tension roller 23 is disposed between supporting roller 22b and backup roller 24. Since tension roller 23 applies a tension toward intermediate transfer belt 21, it presses the belt in the direction shown by the dashed arrowhead line in FIG. 1. As a result of the applied tension, intermediate transfer belt 21 becomes inflated in the direction of the dashed arrowhead line.
In FIG. 1, intermediate transfer belt 21 is horizontally disposed in the primary transfer part and backup roller 24 is disposed under the primary transfer part and between driving roller 22a and supporting roller 22b. Consequently, the intermediate transfer belt unit extends in the left and the right directions resulting in a large intermediate transfer belt unit.
Since fixing device 30 is disposed down stream of and under the intermediate transfer belt unit and recording media transporting belt 29 moves between secondary transfer roller 25 and backup roller 24, if the intermediate transfer unit becomes large in size, the image forming apparatus itself becomes large in size. That is to say that the image forming apparatus shown in FIG. 1 will require a larger image forming apparatus casing 32 as the intermediate transfer belt unit becomes larger in size. As a result, unused or dead space in the casing increases. Furthermore, as intermediate transfer belt 21 needs at least four rollers (i.e., driving roller 22a, supporting roller 22b, tension roller 23, backup roller 24), as well as the primary transfer roller, the apparatus inevitably becomes heavy.
Apart from that, since backup roller 24 is disposed under the primary transfer part and between driving roller 22a and supporting roller 22b, while tension roller 23 is disposed between supporting roller 22b and backup roller 24, the angle created between intermediate transfer belt 21 and recording media transporting belt 29 is small and makes it difficult to separate the recording media from intermediate transfer belt 21. Due to the difficulty of separating the recording media, a media separating mechanism (not shown) needs to be provided.
As seen from the design of the image forming apparatus in FIG. 1, no consideration is given to the compactness of the design nor to the separation of recording media. Meanwhile, Japanese laid-open patent publication JP 1996-87151 discloses an image forming apparatus without an intermediate transfer belt. Accordingly, the apparatus lacks advantages provided by an image forming apparatus with an intermediate transfer belt, such as, obtaining images without color drift and accepting various forms of media (e.g., thin paper, thick paper, transparent material or label paper) without restriction. Furthermore, the direct transfer operation of JP 1996-87151 forces a design limitation where a fixing device needs to be on the same line as a transporting pathway for recording media.
As described earlier, driving roller 22a, supporting roller 22b, tension roller 23, backup roller 24 and the primary transfer roller are supported on the supporting frame to compose the intermediate transfer belt unit, which supports endless intermediate transfer belt 21. Endless intermediate transfer belt 21 also transports the recording medium.
Intermediate transfer belt 21 can not be mounted to the intermediate transfer belt unit, unless the outer circumference of the surface of the intermediate transfer belt unit, which contacts the intermediate transfer belt, is smaller than the inner circumference of intermediate transfer belt 21. When mounting intermediate transfer belt 21, workers must be careful not to damage the surface of intermediate transfer belt 21.
For this reason, a large difference in dimension between the outer circumference of the surface of the intermediate transfer belt unit and the inner circumference of intermediate transfer belt 21 needs to be provided. A tension roller is necessary to remove distortion caused by this large dimensional difference. However, providing the tension roller requires additional mounting space and a mechanism for applying tension. This further contributes to unused space and complicates the structure. The structure becomes heavier due to the increased number of component members, which ultimately leads to increased costs.
The intermediate transfer belt unit has a protective cover mounted in the unit for protecting edges of the intermediate transfer belt. The protective cover is mounted after the intermediate transfer belt is mounted. Accordingly, when the intermediate transfer belt is replaced for maintenance, or when the belt unit is recycled, the protective cover has to be removed in advance. This procedure restricts and complicates maintenance.
To overcome such a drawback, Japanese laid-open patent publication JP1996-123294 discloses an image forming apparatus having a cartridge of an intermediate transfer belt. The cartridge is provided with a supporting member for changing a belt, wherein a tension roller of the intermediate transfer belt is made to be movable against a spring so as to support the intermediate transfer belt. This allows the tension roller to move towards a driving roller when changing the intermediate transfer belt. Thus, the intermediate transfer belt of the apparatus is changeable as a whole cartridge having the supporting member for changing a belt.
However, the apparatus disclosed in Japanese laid-open patent publication JP1996-123294 requires changing a whole cartridge when changing the belt. Not only is the structure complicated but it is also uneconomical and environmentally unfriendly. When changing the belt, the supporting member has to be discarded with the belt.
Another conventional apparatus is disclosed in Japanese patent publication JP3175631 (corresponds to JP1998-260593). The image forming apparatus has a secondary transfer device under and opposite the side of a fixing device from the center of the longitudinal direction of an intermediate transfer belt so as to provide enough distance between the secondary transfer device and the fixing device. There is also a roller for restricting meandering of the belt so as to keep an appropriate tension of the intermediate transfer belt.
However, the additional roller for restricting meandering of the belt in Japanese patent publication JP3175631 enlarges the running region for the intermediate transfer belt and makes it difficult to design a compact apparatus. This inevitably leads to increased costs for the apparatus. Furthermore, when having to change the endless intermediate transfer belt, a cleaning device which cleans paper powder, waste toner or other deposits remaining on the belt is required to be replaced as well.
Ultimate elongation of the belt in the periphery direction needs to be at a minimum. Consequently, an appropriate thickness of the intermediate transfer belt is required, which necessitates hardness of the belt itself. The resultant stiff belt brings about image defects such as an image without midsection. If a rate difference is given between that of an image bearing body and the intermediate transfer body, apparent malfunction of transportation of the belt or color drift occurs. Although an attempt to minimize ultimate elongation of the belt using a material having a high tensile strength such as polyimid resin is done, the material is expensive and hard in its property so that a large nip is necessary when transferring and a high powered motor is necessary to drive the belt accurately.
Japanese laid-open patent publication JP1998-240020 describes an intermediate transfer belt wherein a fibrous substrate is accompanied in rubber to minimize an ultimate elongation of the belt. In addition, a releasing layer is provided on the surface. Though the belt makes a certain level of progress in a point of elongation, soft rubber material is scraped by friction with the driving roller to affect driving performance. In addition, when running for long periods of time the belt will expand and ultimately cause color drift to occur.